This invention relates to color television receivers and in particular to those which incorporate synchronous demodulators. The typical television receiver has a tuner which frequency converts the received information bearing signal by the familiar heterodyning process. A local oscillator is adjusted to produce an oscillatory signal a given frequency above that of a received television signal for converting the sound and video carriers in the received signal to corresponding intermediate frequency carriers which are supplied to frequency selective intermediate frequency amplifiers. The output of these amplifiers drives a detector which recovers the modulation from the carriers.
Due to its close frequency relationship to the suppressed chrominance subcarrier sidebands, the sound carrier in receivers not employing synchronous demodulation must be drastically attenuated in the video IF channel prior to detection of the video IF signal to preclude the production of the well known 920 kHz beat resulting from the presence in the detector of the chrominance and sound information. The effect of this interference on the displayed picture is highly objectionable and such color receivers generally include separate detectors for the luminance-chrominance information, and for the sound information.
The separate detection permits substantial trapping or attenuation of the sound information in the luminance-chrominance channel and minimization of the chrominance-sound beat. The sound trap is typically located in the frequency selective portion of the intermediate frequency amplifier at a point after the sound information has been coupled to the sound detector. The arrangement yields satisfactory reproduction of the televised picture provided the frequency conversion of the tuner is precise enough to insure accurate positioning of the sound carrier within the sound trap. However, significant limitation on the degree of mistuning which may be tolerated is imposed.
While "exact tuning" of a television receiver generally suffices, it is often desirable to adjust the tuner and alter the frequency of the intermediate frequency signal. For example, preferential adjustment of the picture characteristics may be obtained by changing the effect of the intermediate frequency amplifiers on the luminance components or on local extraneous interference signals; or relaxed tuning requirements may be obtained.
As is well known, mistuning of the tuner oscillator moves the intermediate frequency signals, and their corresponding modulation components, within the intermediate frequency filter response characteristic. In receivers employing conventional envelope-type detectors this results in severe chrominance-sound (920 kHz) beat in one direction and loss of color in the other direction.
With currently used highly selective non-linear sound traps phase distortions are produced in signals coupled through them. These phase distortions are, of course, of little significance for the sound information being trapped. However, for chrominance information, which must be accurately reproduced in both phase and amplitude, the effects of these phase distortions are highly objectionable in the displayed picture.
Synchronous demodulators achieve significant reduction in the amount of chrominance-sound beat and are distinguishable from the more conventional envelope demodulators in that they are gated or switched at the carrier frequency by a separate reference carrier. They require close frequency correlation between the demodulator switching signal and the IF carrier.
A television receiver with a synchronous demodulator, as described in the above mentioned U.S. Pat. No. 3,760,094, includes a fixed reference oscillator which produces a reference signal, free of harmonics and modulation components for switching the detectors. Such synchronous demodulators minimize the chrominance-sound beat to such an extent that sound trapping is not required and the television system of the patent does not include sound trapping in the IF amplifier. It does, however, include a frequency control system, operative on the receiver tuner, for maintaining the intermediate frequency signal at the same frequency as the reference oscillator. Therefore, no significant mistuning of the tuner oscillator, whether for preferential tuning reasons or for relaxed tuning requirements, is possible with the system of the above mentioned patent.
Another television receiver system with a synchronous demodulator, described in copending application (Skerlos II), includes a variable frequency reference oscillator to switch the detectors. The reference signal is maintained, by the action of a closed loop APC system, in frequency synchronization and at a predetermined phase with the IF carrier despite its frequency variations. The receiver includes an intermediate frequency amplifier having a response curve which is not distorted by the presence of sound carrier trapping and thus permits substantial deviations in the IF signal frequency without introducing objectionable color distortion or 920 kHz sound beat. The resulting advantage is two-fold. Firstly, the viewer may be provided with a control for preferentially adjusting the characteristic of the color picture reproduced on the receiver without introducing noticeable distortion therein, and secondly the receiver produces an acceptable color picture even though not "accurately tuned". The television receiver may be corrected to a conventional "exact tuning" receiver if desired by incorporating one of the many currently used AFC systems, which however, require additional components that must be properly aligned.